We found this sophisticated meter circuit in an article in Radio Electronics magazine from June 1959. The article taught how to assemble it. Transistors are common types at the time.
This circuit, from a 303 Circuits from the 90s, uses a telephone pickup coil as a sensor to...
Found in a 90s Popular Electronics, this circuit keeps an LED on when a phone is in use. Can be...
The integrated circuit used in this project is not common today. It was found in a 2000...